Thermodynamics of depolymerization of biotechnological poly[(R)‐3‐hydroxybutyrate] with formation of (R,R,R)‐4,8,12‐trimethyl‐1,5,9‐trioxadodeca‐2,6,10‐trione and of its polymerization with ring‐opening to highly isotactic poly[(R)‐3‐hydroxybutyrate] from 0 to 500 K at standard pressure